The invention relates to power amplifiers, and particularly to audio amplifiers adapted to be coupled in bridged mode to a speaker load and driven by a separate source amplifier.
Bridging stereo or two channel amplifiers is known. See, for example, FIG. 1 which illustrates a known bridged amplifier 10 comprising a first amplifier 12 having an input and an output and a second amplifier 14 likewise having an input and an output. The amplifiers 12 and 14 are identical and are coupled across opposite terminals or sides of a load such as a loud speaker 16. The input of amplifier 12 is coupled to the input of the amplifier 14 through a negative unity gain amplifier 18. Bridging an amplifier thus consists of sending an inverted input signal (commonly referred to as reverse phase or more correctly reverse polarity) to one channel, and connecting the speaker load to the active terminals of each amplifier. The speaker sees an alternating current (AC) signal that swings positive in one terminal (amplifier channel 12) and swings negative on the other terminal (amplifier channel 14) instead of the more typical connection of one speaker terminal connected to ground and the other to an active signal source.
The advantage that bridging a two channel amplifier offers is increased power output, theoretically quadruple the power output or 6 dB over normal mode of stereo or independent two channel output. This occurs without increasing the power supply rails, a common limitation due to limited output device voltage and current at voltage ratings.
In applications where the power supply voltage is limited, bridging multiple amplifiers is an effective way to generate more power than can ordinarily be derived. For example, car stereos, and particularly add-on car stereo amplifiers use amplifier bridging to great advantage. However, car stereos often employ auxiliary power supplies to achieve the required power levels.
Typically, the conventional approach to bridging two identical power amplifiers involves providing the extra circuitry inside the two channel amplifier, and utilizing a switch to change from independent two channel mode to bridged mode. Then, instead of the speaker load being coupled across the active signal output terminal of each channel and ground, the load is connected across the two active signal output terminals, with no connection of the speaker to ground. A stereo amplifier is essentially reduced to a single channel output when bridged. Therefore, if two channels are required for the final application, two more channels, or another stereo amplifier, must be added to have two bridged channels of increased power.
When an amplifier is connected in bridge mode, each channel sees the reverse polarity output of the other channel through the speaker or load. Reverse polarity loading draws more current from each channel, which is where the extra power is derived. As noted above, an increase of 6 dB is possible, which is a quadrupling of the power output. This 6 dB increase incidentally is achieved only if the load impedance stays the same as it was for the nominal two channel operation. Thus, if an amplifier is capable of driving a four ohm load when connected in independent two channel mode, the same amplifier can only handle an 8 ohm load when used in the bridge mode. Accordingly, the effective increase in the power output is about 3 dB, which is a doubling of the output power. In essence, bridging the amplifiers has the result of adding together the output power of each amplifier.
Normally two different amplifiers, i.e., amplifiers in separate chassis or enclosures, may not be added together to sum their power. Typically, if the output of one power amplifier is coupled to the input of another power amplifier, the input from the first amplifier is treated as a voltage signal and the power of the first amplifier is not added to the power of the second amplifier. It is only the power of the second amplifier that is available to the speaker load. See, for example, the system 20 in FIG. 2 in which a source amplifier 22, which may be on the sound card 23 of a computer 25, has its output coupled to the input of a second amplifier 24 which is in turn coupled to a speaker 26. If bridging of the two amplifiers 22 and 24 were attempted, unless the two amplifiers are closely matched in power output capability, there would exist the possibility of damage to the lesser powered amplifier. For example, if a 2 watt amplifier is bridged to a 35 watt amplifier, the more powerful amplifier will overpower the smaller one. Not only would the smaller amplifier run out of power or clip the signal before the high power amplifier, but the resulting distortion of the signal would cause the larger amplifier to sound as if it were distorted as well. Thus, any benefit from the bridging would be limited to the power level of the smaller amplifier. This would severely limit the usefulness of connecting amplifiers in bridged mode.
Some situations are more well defined. For example, car stereos and computer sound cards both have a relatively well defined power supply voltage of about 12 VDC. This defines and thus limits how much power can be derived by a power amplifier supplied from such voltage (approximately 4 watts clean and about 5 watts at 10% distortion) without bridging or some sort of voltage step-up power supply, both of which are more costly and complex.
In the case of computer sound cards, the standard one-eighth inch stereo output jack is ground referenced and both channels have a common ground. This effectively prevents using a bridged power amplifier output approach for the sound card, because a bridged mode for stereo channels requires that both channel outputs not be grounded at either output terminal, and they may not be connected together. Computer sound cards must therefore operate with either directly derived power amplifier supply voltage, which is internally limited to about 12 volts DC, or a voltage step-up type power supply. Voltage step-up type power supplies draw excessive current which can adversely affect the 12 volt power supply of the computer. Also, due to inefficiency in voltage step-up conversion, attempting to step-up the +5 VDC power rail used to power the digital logic circuits in the computer and in memory is not a practical or cost effective expedient.
In a powered speaker application, it is common to electrically equalize the output of the amplifier to compensate for and adjust the frequency response of the speaker in the enclosure. In the normal application of bridged amplifiers, it is not typical to apply equalization to one or the other of the amplifier channels directly. Equalization is normally applied earlier in the signal chain. However, it is not always possible to apply equalization prior to the sound card amplifier. Often, the external amplifier takes the signal after it has gone through the sound card amplifier. Some systems such as the so-called Creative Labs Sound Blaster series as well as the Media Vision series of cards provide for tone controls in the software provided with the cards. For example, the Windows application has such capability. However, some users may not employ Windows, or may have a sound card that does not provide such tonal control capability. Therefore, it is not assured that such capability is always available. Further, it is not always preferable to have the end user set the tone controls to achieve an intended flat response.
It is desirable to be able to increase the power available to drive a load from a source of limited supply, for example, a computer sound card. It is desirable to employ such a limited power device to drive a powered speaker system which is separate and distinct from the computer or other source device so as to achieve a desirable sound level. It is also desirable to provide equalization to multimedia speakers, Special selectable and modular units are also desirable.